System and method for the processing of lng

ABSTRACT

A system for processing an LNG feed, the system comprising: a bulk removal stage arranged to remove and release CO2 liquid from the inflow feed, said bulk removal stage including a first HGMT device, and; a polishing stage arranged to receive a lean CO2 feed from the first HGMT device, said polishing stage arranged to remove and release residual CO2, the polishing stage including a second HGMT device; wherein the polishing stage is arranged to release an outflow of CO2 stripped LNG.

FIELD OF THE INVENTION

The invention relates to the processing of LNG from a production field. In particular, the invention relates to the removal of adverse gases including CO₂, H₂S and N₂ from a field originating feed stream

BACKGROUND

In order to manage the low quality of natural gas for a field, which is laden with impurities such as high CO₂, H₂S and N₂, typically requires a complex cryogenic separation process the separation to meet product specifications. Cryogenic separation is normally associated with tall column and heavy weight to meet the separation target of sales gas/LNG specs.

Alternatively, solvent-based removal processes for pre-treatment of nitrogen involve high energy consumption.

Both such processes require high CAPEX, OPEX, weight and footprint when applied to offshore conditions.

SUMMARY OF INVENTION

In a first aspect, the invention provides a system for processing an LNG feed, the system comprising: a bulk removal stage arranged to remove and release CO₂ liquid from the inflow feed, said bulk removal stage including a first HGMT device, and; a polishing stage arranged to receive a lean CO₂ feed from the first HGMT device, said polishing stage arranged to remove and release residual CO₂, the polishing stage including a second HGMT device; wherein the polishing stage is arranged to release an outflow of CO₂ stripped LNG.

In a second aspect, the invention provides a method for processing an LNG feed, the method comprising the steps of: separating and releasing CO₂ liquid from the inflow feed using a first HGMT device; receiving a lean CO₂ feed from the first HGMT device at a second HGMT device; polishing said lean CO₂ feed and releasing residual CO₂ using the second HGMT device, and; releasing an outflow of CO₂ stripped LNG.

In a third aspect, the invention provides a system for processing an LNG feed, the system comprising: an N₂ removal stage arranged to remove and releasing N₂ liquid from the inflow feed, said N₂ removal stage including an N₂ separation HGMT device, and; wherein the N₂ separation HGMT device is arranged to release an outflow of N₂ stripped LNG.

The development of both CO₂ and N₂ separation technologies will enable the monetisation of undeveloped gas fields having a high level of impurities. However, relying upon high cost, and large footprint, processes such as column distillation lessen the economic and technical viability of such fields.

High gravity mass transfer (HGMT) devices solve that part of the process, but the incorporation of HGMT devise so as to optimise the removal of such impurities remains key. HGMT devices eliminate the need for solvent based processes for conventional acid gas removal, as well as the necessity for excessive pre-cooling prior to entering the cryogenic process, as well as the requirement for additional dehydration units for pre-treatment, particularly for N₂ removal.

BRIEF DESCRIPTION OF DRAWINGS

It will be convenient to further describe the present invention with respect to the accompanying drawings that illustrate possible arrangements of the invention. Other arrangements of the invention are possible and consequently, the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding description of the invention.

FIG. 1 is a schematic view of a CO₂ removal system according to one embodiment of the present invention;

FIG. 2 is a schematic view of an N₂ removal system according to a further embodiment of the present invention;

FIG. 3 is a schematic view of a combined CO₂ and N₂ removal system according to a further embodiment of the present invention; and

FIG. 4 is a schematic view of a generalised high gravity mass transfer device according to a further embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 shows a schematic view of a CO₂ removal system 5 according to one embodiment of the present invention. The system 5 receives an inflow of a feed stream From natural gas stream, which may, for instance have more than 40% CO₂ content, less than 15% N₂ and including a portion of H₂S. This is delivered to a high gravity mass transfer device 10 which in this case forms the basis for a bulk removal and polishing stage. The intention of the first HGMT 10 is to remove the majority of CO₂ and H₂S from the feed stream 20, to release 32 liquid CO₂ from an outlet 30 in the HGMT 10. This first bulk removal may represent from 90 to 95% of the CO₂ present in the feed stream. Meanwhile, stream 35 returned to HGMT 10 to further refine the bulk removal stage. The feed stream having lean CO₂ is then directed 25 to a polishing stage having a second HGMT 15. The feed 25 may release a maximum of 14% CO₂, and a proportionally increased N₂ component. The feed 25 is cooled by a heat exchanger 42 and passed to a vessel 40 in which the liquid CO₂ is returned 45 to HGMT 10. A portion 48 of the liquid is cooled and expanded in a heat exchanger 44 to a lower pressure prior to passing to an upper section of second HGMT 15. Meanwhile, the lean-CO₂ vapour 50 will heat up in a further heat exchanger 46 and expand before entering lower section of HGMT 15. HGMT 15 must always operate at lower pressure than HGMT 10 however at maximum operating pressure of 40 bar.

Residual CO₂ is removed from the feed stream and returned 65 to the first HGMT 10 via pump for the hydrocarbon recovery from HGMT 15. Stream 65 rich in CO₂ introduces to the HGMT 10 will avoid solid CO₂ solid region.

The polished feed stream having the substantial CO₂ component removed therefrom is directed and cooled 60 to a vessel 70. The liquid 75 will be pump back to HGMT 15 to further enhance the separation. The CO₂ lean stream 80 contains minimal CO₂ content of at least 50 ppm with a proportionally increased N₂ content in hydrocarbon rich stream.

In this further embodiment, the polishing stage receives the feed stream under cryogenic conditions, although cryogenic conditions used in this embodiment is slightly different than conventional definition cryogenic conditions. To this end, a refrigeration unit 55 which may use for instance liquid nitrogen is directed to the heat exchangers leading into the HGMT 15 to cool down the process streams to meet the cryogenic operating conditions.

FIG. 2 shows a further aspect of the present invention whereby an N₂ removal stage 85 includes an HGMT device 90 for receiving a feed stream 95 that is nitrogen laden. The nitrogen laden gas passes through a preconditioning stage 100, to cool down the stream 105 temperature to the cryogenic temperature of −60° C. to −120° C., into the HGMT device 90. The HGMT device 90 according to this embodiment, has been modified, in that internal housing elements, rotating parts, apparatus to avoid flow maldistribution, etc., have been modified from conventional HGMT devices, so as to meet the low temperature condition and produce on-spec LPNG product.

A separator vessel 125 is separates a vapour component 130 and liquid component 127. A nitrogen rich stream containing at least 97% nitrogen is drawn off from overhead the vessel 125. The liquid component 127 is passed to HGMT device 90 via pump to further enhance the liquid outflow 110 which contains mostly liquid hydrocarbon with very minimal CO₂ and N₂ content. This component passes through a heat exchanger and is drawn off 85 as the LNG product. A return 120 stream is directed back to the HGMT device 90 for further separation.

FIG. 3 shows a system 140 of an integrated HGMT process for CO₂ and N₂ removal from natural gas to LNG production. The integrated system 140 combines a multi stage separation process whereby a feed stream 160 is passed into the HGMT 145 for bulk removal of CO₂ and subsequent disposal of liquid CO₂ rich stream 180 with minimal hydrocarbon content. The lean CO₂ stream from the HGMT 145 is directed 165 to the second polishing stage of the second HGMT 150 to meet the utmost CO₂ content of 50 ppm in the 170 stream. A proportionally increased of N₂ content in stream 170 is directed to a third HGMT 155 for the nitrogen removal to produce LNG product of stream 185 (contains less than 1% of nitrogen). Furthermore, the nitrogen rich gas stream 190 is drawn off at the overhead section of HGMT 155.

This invention can be further employed to meet the nitrogen content in the fuel gas stream for the self-consumption with minimal impact to LNG production.

FIG. 4 shows a generalized view of an HGMT device 200 comprising a high gravity mass transfer device housing which includes a rotational part 207, an internal cavity 211 for an arranged elements to enhance contact area for gas and liquid phases and one or more quantity apparatus can be installed in the housing to minimise the flow maldistribution inside the HMGT device.

Two phase feed stream 215 is introduce into a chamber 205 having a rotating elements 207. The arranged elements experienced the rotational movement resulting in high centrifugal force producing smaller liquid droplets amplified the mass transfer and heat transfer with higher overall separation efficiency and shorter residence time. Under this condition, most of the gas will be diverted to centre of the arranged elements due to the difference in velocity and drawn off at the overhead section 222 of HGMT device. Meanwhile, liquid product stream 224 is produced at the bottom section of HGMT device. A liquid stream 220 from an overhead vessel is diverted back to HGMT device to assist the separation process. 

1-14. (canceled)
 15. A system for processing an LNG feed, the system comprising: a bulk removal stage arranged to remove and release CO₂ liquid from the inflow feed, said bulk removal stage including a first HGMT device, and a polishing stage arranged to receive a lean CO₂ feed from the first HGMT device, said polishing stage arranged to remove and release residual CO₂, the polishing stage including a second HGMT device; wherein the polishing stage is arranged to release an outflow of CO₂ stripped LNG.
 16. The system according to claim 15, further including a first heat exchanger arranged to receive a portion of the lean CO₂ feed, said first heat exchanger arranged to cool and expand the lean CO₂ feed to a lower pressure prior to directing said lean CO₂ feed to the second HGMT device.
 17. The system according to claim 16, further including a second heat exchanger arranged to receive a vapour portion of the lean CO₂ feed, said second heat exchanger arranged to heat and expand said vapour portion before entering the second HGMT device.
 18. The system according to claim 15, wherein the second HGMT device is further arranged to separate and direct a portion of the CO₂ stripped LNG to the first HGMT device.
 19. The system according to claim 15, further including a refrigeration system for cooling the inflow from the bulk removal stage prior to entering the second HGMT device.
 20. The system according to claim 15, further including a reflux vessel for separating a portion of the inflow from the bulk removal and re-directing to the HGMT device before entering the polishing stage.
 21. A method for processing an LNG feed, the method comprising the steps of: separating and releasing CO₂ liquid from the inflow feed using a first HGMT device; receiving a lean CO₂ feed from the first HGMT device at a second HGMT device; polishing said lean CO₂ feed and releasing residual CO₂ using the second HGMT device, and; releasing an outflow of CO₂ stripped LNG.
 22. The method according to claim 21, further including the step of directing a portion of the CO₂ stripped LNG to the first HGMT device from the second HGMT.
 23. The method according to claim 21, further including the step of cooling the bulk separated inflow prior to entering the second HGMT device.
 24. The method according to claim 21, further including the steps of: cooling and expanding a liquid portion of the lean CO₂ feed by a first heat exchanger, and so; lowering the pressure of lean CO₂ feed, then; directing said lean CO₂ feed to the second HGMT device.
 25. The method according to claim 24, further including the steps of: heating and expanding a vapour portion of the lean CO₂ feed using a second heat exchanger, then; directing the vapour portion of the lean CO₂ feed to the second HGMT device.
 26. A system for processing an LNG feed, the system comprising: an N₂ removal stage arranged to remove and releasing N₂ liquid from the inflow feed, said N₂ removal stage including an N₂ separation HGMT device, and wherein the N₂ separation HGMT device is arranged to release an outflow of N₂ stripped LNG.
 27. A method for processing an LNG feed, the system comprising: removing and releasing N₂ liquid from the inflow feed, using an N₂ separation HGMT device, and releasing an outflow of N₂ stripped LNG.
 28. A system for processing an LNG feed, the system comprising; the bulk removal stage and the polishing stage according to claim 1; said polishing stage arranged to release an outflow of CO₂ stripped LNG to the N₂ removal stage according to claim
 12. 29. The system according to claim 16, wherein the second HGMT device is further arranged to separate and direct a portion of the CO₂ stripped LNG to the first HGMT device.
 30. The system according to claim 17, wherein the second HGMT device is further arranged to separate and direct a portion of the CO₂ stripped LNG to the first HGMT device. 